The Messenger of Allah ﷺ used to give to some of the detachments he sent out (something extra) for themselves in particular apart from the division made to the whole army. The fifth is necessary in all that. (Using translation from Abū Dāʾūd 2746)
أَنَّ رَسُولَ اللهِ ﷺ كَانَ يُنَفِّلُ بَعْضَ مَنْ يَبْعَثُ مِنَ السَّرَايَا لِأَنْفُسِهِمْ خَاصَّةً سِوَى قَسْمِ عَامَّةِ الْجَيْشِوَالْخُمُسُ فِي ذَلِكَ وَاجِبٌ لِلَّهِ تَعَالَى